The observed γray fluence distribution of terrestrial gamma ray flashes (TGFs) detected by the Fermi Gamma‐ray Burst Monitor (GBM) is altered by instrumental effects. We perform corrections for dead ...time, pulse pileup, and detection efficiency in a model‐independent manner. A sample of 106 GBM TGFs is selected to include both TGFs that triggered GBM and weaker TGFs found using an off‐line search. Detector dead time and pulse pileup lower the observed fluence of each TGF and the detection efficiency causes weaker TGFs to have a lower probability of detection than brighter TGFs. Monte Carlo simulations are performed in each case to correct for these effects. The corrected fluence distribution is well fit with a power law of index α=−2.20±0.13. This is consistent with previous estimates using other techniques. Neither a high‐fluence cutoff nor a low‐fluence limit is found. The fluence distribution is also expressed in units of TGF h−1 km−2 versus photons cm−2 per TGF.
Key Points
Determine and correct for GBM instrumental effects on TGFs
To determine the true TGF photon fluence distribution at Fermi altitude
To estimate the number of TGFs per year under the orbit of Fermi
Aims. In the past four decades, it has been observed that solar flares display quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest, i.e. gamma-ray, frequencies in the ...electromagnetic spectrum. It remains unclear which mechanism creates these QPPs. In this paper, we analyze four bright solar flares that display compelling signatures of quasi-periodic behavior and were observed with the Gamma-Ray Burst Monitor (GBM ) onboard the Fermi satellite. Because GBM covers over three decades in energy (8 keV to 40 MeV), it is regarded as a key instrument in our attempt to understand the physical processes that drive solar flares. Methods. We tested for periodicity in the time series of the solar flares observed by GBM by applying a classical periodogram analysis. However, in contrast to previous authors, we did not detrend the raw light curve before creating the power spectral density (PSD) spectrum. To assess the significance of the frequencies, we used a method that is commonly applied to X-ray binaries and Seyfert galaxies. This technique takes into account the underlying continuum of the PSD, which for all of these sources has a P(f) ~ f−α dependence and is typically labeled red-noise. Results. We checked the reliability of this technique by applying it to observations of a solar flare that had been observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI ). These data contain, besides any potential periodicity from the Sun, a 4 s rotational period caused by the rotation of the spacecraft about its axis. We were unable to identify any intrinsic solar quasi-periodic pulsation but we did manage to reproduce the instrumental periodicity. Moreover, with the method adopted here, we do not detect significant QPPs in the four bright solar flares observed by GBM. We stress that for this kind of analyses it is of utmost importance to account appropriately for the red-noise component in the PSD of these astrophysical sources.
The Gamma‐Ray Burst Monitor (GBM) on the Fermi Gamma‐Ray Space Telescope (Fermi) detected 50 terrestrial gamma‐ray flashes (TGFs) during its first 20 months of operation. The high efficiency and ...large area of the GBM detectors, combined with their fine timing capabilities and relatively high throughput, allow unprecedented studies of the temporal properties of TGFs. The TGF pulses are observed to have durations as brief as ∼0.05 ms, shorter than previously measured. There is a relatively narrow distribution of pulse durations; the majority of pulses have total durations between 0.10 and 0.40 ms. In some TGF events, risetimes as short as ∼0.01 ms and falltimes as short as ∼0.03 ms are observed. Three of the 50 TGFs presented here have well‐separated, double peaks. Perhaps as many as 10 other TGFs show evidence, to varying degrees, of overlapping peaks. Weak emission is seen at the leading or trailing edges of some events. Five of the 50 TGFs are considerably longer than usual; these are believed to be caused by incident electrons transported from a TGF at the geomagnetic conjugate point. TGF temporal properties can be used to discriminate between models of the origin of TGFs and also provide some basic physical properties of the TGF process.
Zebrafish are an existing model for genetic and developmental studies due to their rapid external development and transparent embryos, which allow easy manipulation and observation of early ...developmental stages. The application of the zebrafish model to vision research has allowed for examination of retinal development and the characteristics of different retinal cell types, including bipolar cells. In particular, bipolar cell development, including differentiation, maturation, and gene expression, has been documented, as has physiological properties, such as voltage- and ligand-gated currents, and neurotransmitter receptor and ion channel expression. Mutant strains and transgenic lines have been used to document how bipolar cell connections and/or development may be altered, and toxicological studies examining how environmental factors may impact bipolar cell activity have been performed. The purpose of this paper was to review the existing literature on zebrafish bipolar cells, to provide a comprehensive overview of current information pertaining to this retinal cell type.
ABSTRACT We have performed a blind search for a gamma-ray transient of arbitrary duration and energy spectrum around the time of the LIGO gravitational-wave event GW150914 with the six-spacecraft ...interplanetary network (IPN). Four gamma-ray bursts were detected between 30 hr prior to the event and 6.1 hr after it, but none could convincingly be associated with GW150914. No other transients were detected down to limiting 15-150 keV fluences of roughly 5 ×10−8-5 × 10−7 erg cm−2. We discuss the search strategies and temporal coverage of the IPN on the day of the event and compare the spatial coverage to the region where GW150914 originated. We also report the negative result of a targeted search for the Fermi-GBM event reported in conjunction with GW150914.
GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ...Robotic Optical Transient Search Experiment (ROTSE) and a host of other telescopes, and was detected in the radio by the Very Large Array. The redshift of z= 3.355 ± 0.005 was determined by Keck/High Resolution Echelle Spectrometer (HIRES) and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3–103 keV, systematically softens over time, with Epeak moving from ∼600 keV at the start to ∼40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasi-thermal model shifting from ∼60 to ∼3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this is originated from the afterglow component, not from the pulsed/flaring prompt emission. Broad-band optical and X-ray coverage of the afterglow at the start of the final X-ray decay (∼8 ks) reveals a spectral break between the optical and X-ray bands in the range of 1015–2 × 1016 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ∼3 × 105 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3 × 1053 and 1.6 × 1052 erg for the afterglow; there is no evidence for a jet break in the afterglow up to 6 d following the burst.
Abstract The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors ...show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1–2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.
Whole-cell patch recording and puff pipette techniques were used to identify glutamate receptor mechanisms on bipolar cell
(BC) dendrites in the zebrafish retinal slice. Recorded neurons were stained ...with Lucifer Yellow, to correlate glutamate responses
with BC morphology.
BC axon terminals (ATs) consisted of swellings or varicosities along the axon, as well as at its end. AT stratification patterns
identified three regions in the inner plexiform layer (IPL): a thick sublamina a , with three bands of ATs, a narrow terminal-free zone in the mid-IPL, and a thin sublamina b , with two bands of ATs. BCs occurred with ATs restricted to sublamina a (Group a ), sublamina b (Group b ) or with ATs in both sublaminae (Group a/b ).
OFF-BCs belonged to Group a or Group a/b . These cells responded to glutamate or kainate with a CNQX-sensitive conductance increase. Reversal potential ( E rev ) ranged from â0.6 to +18 mV. Bipolar cells stimulated sequentially with both kainate and glutamate revealed a population
of glutamate-insensitive, kainate-sensitive cells in addition to cells sensitive to both agonists.
ON-BCs responded to glutamate via one of three mechanisms: (a) a conductance decrease with E rev â 0 mV, mimicked by L-(+)-2-amino-4-phosphonobutyric acid (APB) or trans -1-amino-1,3-cyclopentanedicarboxylic acid ( trans -ACPD), (b) a glutamate-gated chloride conductance increase ( I Glu -like) characterized by E rev ⥠E Cl (where E Cl is the chloride equilibrium potential) and partial blockade by extracellular Li + /Na + substitution or (c) the activation of both APB and chloride mechanisms simultaneously to produce a response with outward
currents at all holding potentials. APB-like responses were found only among BCs in Group b , with a single AT ramifying deep within sublamina b ; whereas, cells expressing I Glu -like currents had one or more ATs, and occurred within Groups b or a/b .
Multistratified cells (Group a/b ) were common and occurred with either ON- or OFF-BC physiology. OFF-BCs typically had one or more ATs in sublamina a and only one AT in sublamina b . In contrast, multistratified ON-BCs had one or more ATs in sublamina b and a single AT ramifying deep in sublamina a . Multistratified ON-BCs expressed the I Glu -like mechanism only.
Visual processing in the zebrafish retina involves at least 13 BC types. Some of these BCs have ATs in both the ON- and OFF-sublaminae,
suggesting a significant role for ON- and OFF-inputs throughout the IPL.
Current space‐based observations of terrestrial gamma ray flashes (TGFs) are capable of identifying only TGFs that exceed a lower brightness threshold. Observationally, dim TGFs that fall below this ...threshold are consequently difficult to find using photon‐only search algorithms. Such TGFs are a potentially important part of the overall global TGF rate, and information on their occurrence rate would give important insight into TGF generation mechanisms. We describe and implement a lightning‐based search for TGFs that uses the location and time of National Lightning Detection Network reported positive polarity, in‐cloud (+IC) discharges of the type known to be directly associated with TGFs. These events identify a 200 μs search window when any associated TGF photons would have been detected. We show that this approach can detect TGFs without requiring a lower threshold on the detected photon brightness of the event, and thus is capable, in principle, of finding a population of weak TGFs. We find that TGFs occur at a rate between 1 in 40 and 1 in 500 of in‐cloud lightning events that meet our study's criteria. The distribution of gamma ray counts in the search windows exhibits a statistically significant lack of nearby dim TGFs below the GBM search threshold. The data favor a brightness distribution in which nearby observationally dim TGFs are rare.
Key Points
We developed a method to identify TGFs with no lower limit on fluence
The fraction of strong in‐cloud lightning that produces TGFs is estimated to be around 1 in 100
Dim TGFs are likely rarer than expected from extrapolation of the known rate of brighter TGFs
Ultrastructural examination of photoreceptor inner segment ellipsoids in larval (4, 8, and 15 days postfertilization; dpf) and adult zebrafish identified morphologically different types of ...mitochondria. All photoreceptors had mitochondria of different sizes (large and small). At 4 dpf, rods had small, moderately stained electron-dense mitochondria (E-DM), and two cone types could be distinguished: (1) those with electron-lucent mitochondria (E-LM) and (2) those with mitochondria of moderate electron density. These distinctions were also apparent at later ages (8 and 15 dpf). Rods from adult fish had fewer mitochondria than their corresponding cones. The ellipsoids of some fully differentiated single and double cones contained large E-DM with few cristae; these were surrounded by small E-LM with typical internal morphology. The mitochondria within the ellipsoids of other single cones showed similar electron density. Microspectrophotometry of cone ellipsoids from adult fish indicated that the large E-DM had a small absorbance peak (∼0.03 OD units) and did not contain cytochrome-c, but crocetin, a carotenoid found in old world monkeys. Crocetin functions to prevent oxidative damage to photoreceptors, suggesting that the ellipsoid mitochondria in adult zebrafish cones protect against apoptosis and function metabolically, rather than as a light filter.
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